The present invention relates to plasma-assisted chemical vapor deposition reactors and techniques. In one aspect, the invention relates to reactors and methods for performing high rate plasma-assisted etching. In another aspect, the invention relates to reactors and methods for performing high rate plasma-assisted chemical vapor deposition. Particular applications of the invention include cleaning (and self-cleaning) of reactors and chambers; depositing films on wafers; and cleaning/etching of wafers.
Chemical vapor deposition (CVD) of a solid on a surface involves a heterogeneous surface reaction of gaseous species that adsorb onto the surface. The rate of film growth and the film quality depend on the surface temperature and on the gaseous species available. Some films can be deposited at relatively low surface (substrate) temperatures. One example is CVD silicon dioxide, which is deposited at temperatures of about 400.degree.-600.degree. C. However, some other films, such as silicon nitride, have required much higher temperatures. In certain applications, such temperatures are prohibitive.
Perhaps the classic example of the limitations of high temperature chemical vapor deposition involves the use of CVD films for the final passivation layer of integrated circuits. If no passivating film is used, a relatively high cost, hermetically sealed, ceramic IC package is required. Silicon nitride is the most effective passivating film and would permit the use of low cost plastic encapsulation as an alternative to ceramic IC packaging. Unfortunately, the CVD formation of silicon nitride passivation films require substrate temperatures of approximately 800.degree.-900.degree. C. Because the aluminum metallization used in integrated circuits melts at 600.degree. C., CVD silicon nitride could not be used as the final passivating film. However, the application of plasma techniques to assist silicon nitride CVD deposition lowered the required substrate processing temperatures to about 300.degree.-350.degree. C. This made possible the use of silicon nitride as the final passivation film and, thus, made possible high quality plastic IC encapsulation.
More recently, plasma-enhanced low temperature deposition techniques have been used to form other dielectric films such as silicon dioxide interlayer films. The technique may be applicable to the deposition of diverse materials such as aluminum, silicon, iron oxide, refractory metals such as tungsten, and refractory metal silicides.
The plasma used in all of the commercial plasma-assisted CVD reactors is a low pressure, reactant gas discharge developed in an RF field. A plasma is, by definition, an electrically neutral ionized gas in which there are equal densities of electrons and ions. At the low pressures used in plasma-assisted CVD, typically 50 to 1,000 millitorr, the discharge is in the "glow" region and the electron energies can be quite high relative to the heavy particle energies (T.sub.E .perspectiveto.20,000.degree.-30,000.degree. K.). However, charge neutrality is violated and a plasma "sheath" develops in the region of any physical surface bounding the plasma. Such a sheath develops along electrode surfaces. Ions are accelerated within the plasma across the sheath to the electrode surface. The very high electron temperatures create many dissociated species that can recombine on nearby surfaces (such as substrates), allowing a CVD film to grow. This supplying of reactive free radicals and plasma ion bombardment of the film makes possible low-temperature, silicon nitride chemical vapor deposition, and is critical in general to plasma-assisted chemical vapor deposition.
Using plasma-assisted chemical vapor deposition, silicon nitride is formed from silane, nitrogen and ammonia reactants as follows: EQU SiH.sub.4 +NH.sub.3 +N.sub.2 .fwdarw.Si.sub.y N.sub.y H.sub.z ( 1)
The concentration of hydrogen in the deposited silicon nitride may be as high as 35 atomic percent. For final passivation films, the presence of hydrogen probably does not create problems. However, hydrogen diffusion during high temperature fabrication steps can cause non-uniform electrical characteristics in some existing structural features and components.
There are currently available two types of commercially useful plasma deposition reactors. Referring to FIGS. 1-2, these are, respectively, the hot wall barrel reactor 10 and the radial flow reactor 20.
An example of a hot wall plasma-assisted reactor is shown in FIGS. 1A and 1B. The reactor 10 illustrated schematically in FIG. 1A employs an inlet or inlet manifold 11 to supply gas or a mixture of gases to one end of the furnace tube 12, and a vacuum/exhaust outlet 13 at the opposite end which is connected to a vacuum pump (not shown). Deposition is accomplished by a combination of heat provided by electric resistance coils 14 and of reactive species provided locally to the substrates 5--5 by a system 16 of parallel, RF electrodes 17 and 18. The reactor 12 fits into the bore of a diffusion furnace. The electrodes 17 and 18 of system 16 are rectangular and run essentially the length of the furnace tube 12. Alternate electrodes 17 and 18 are connected to the power lead of RF generator 19 and to ground. As the gas mixture flows across the pressure gradient down the length of the tube 12, the electrodes 17 and 18 apply the plasma-generating RF field of 400-500 kilohertz substantially perpendicular to the surface of the substrates. This provides relatively efficient, plasma-assisted deposition of the gaseous species on the heated substrates 5--5.
FIG. 2 schematically illustrates a radial-flow, plasma-assisted CVD reactor 20 which is commercially available from Applied Materials Corporation of Santa Clara, Calif. In one embodiment, the unreacted gases enter the pancake-shaped chamber 21 through an inlet shaft 22 which opens through the center of lower platen or electrode 23. The gas flows radially across the electrode 23 and substrates 5--5 and exits the chamber at outlets 24--24 located at the chamber periphery. The platen 23 is grounded.
RF power is applied to an upper electrode 25 to create a low frequency, 50 kilohertz RF discharge in the gas flow. A magnetic-coupling rotation drive system 27 is provided for rotating the lower electrode/platen 23 to promote uniform deposition. In this system 20, the electrodes 23 and 25 are spaced apart approximately two inches and are approximately twenty-six inches in diameter. This provides a uniform glow discharge and further promotes uniform deposition. In addition, the platen 23 is heated by the heater blocks 26--26 to temperatures of approximately 300.degree. C. to promote recombination and deposition on the substrates. This relatively high temperature reduces hydrogen content (see equation (1), above) and eliminates film cracking. The system 20 is designed for reactor cleaning, controlled wafer etching, and in-situ wafer precleaning and deposition in the same reactor. Specifically, regarding deposition, the radial reactor 20 can deposit silicon nitride, silicon dioxide or amorphous silicon films using a low pressure (200-300 millitorr), low frequency (50 kHz), RF-generated plasma. Examples of applications include oxide for bubble memory interlayers, nitride for GaAs device fabrication, nitride and amorphous silicon nitride for solar cell bulk material and anti-reflective coatings, and numerous nitride passivation applications.
The above-described features of the reactor 20 provide excellent film uniformity and quality at deposition rates of 300-400 angstroms per minute, and provide in-situ wafer cleaning rates of about 1000 angstroms per minute.
Each of the above-described reactors and in general, every deposition chamber forms deposits on internal surfaces including the walls and electrodes during deposition. These surfaces must be cleaned after a number of deposition cycles. Typically this "etch back" is done by disassembling the chamber and immersing the chamber in a wet chemical etchant bath. As is well known in the art, this etch back is time consuming and detracts from production throughput in that the chamber must be disassembled, etched and reassembled. Also, the process is awkward and may produce yield-decreasing particulates and contaminants.
Presumably, each of the reactor systems shown in FIGS. 1 and 2 has the potential for self-cleaning using the system plasma electrodes or coils to generate an etching plasma. In particular, the discovery which led to the present invention resulted from deficiencies in the plasma cleaning of radial flow reactors of the type shown in FIG. 2. For the particular dimensions of reactor chamber 20, a relatively high, uniform cleaning rate of 1,000 angstroms per minute is possible using the system electrodes 23 and 25 to apply RF power of approximately 60 kilohertz to generate an etching plasma from CF.sub.4 and O.sub.2 flowing at 800 sccm and 160 sccm, respectively. Etchback is required after about every three to five one-micron deposition cycles to remove the resulting three to five micrions of unwanted film deposits and restore throughput and yield capability. This plasma dry etch is less awkward, is safer and uses less time than wet chemical etching. Nonetheless, the time required to dry etch (to introduce and stabilize the gases, pump down and etch) can still reduce system hourly throughput by as much as 50 percent.
In addition, plasma self-etching of such reactors may be effective for only a few cleaning cycles in restoring deposition throughput and yield capability. After several plasma cleaning cycles system uniformity goes out of spec, and it is then necessary to use a wet chemical etchback. In addition, if RF power is increased during plasma etching in an attempt to increase the plasma etch rates, wet cleaning is necessary even more frequently.
Thus, the current state-of-the-art of plasa-assisted CVD reactors can be summarized as follows. First, plasma-assisted CVD reactors in general provide maximum processing rates of 300 to 400 angstroms per minute for deposition and 1000 angstroms per minute for etching. These relatively slow etch and deposition rates are offset by the availability of excellent film quality and relatively high throughput using batch processing.
Secondly, plasma self-cleaning of such reactors, while preferred to wet chemical etching, is generally a relatively slow process. As discussed above, increasing power to increase self-etch rates may actually decrease already low throughput.
Thirdly, the use of plasma techniques to clean reactors does not necessarily eliminate the use of wet chemical etching, for wet chemical cleaning may still be required to supplement plasma etching.